The present invention relates to power devices, more particularly to a switching power supply that is configured to operate directly off an AC power source.
The conventional power transistors for switching applications, e.g., Bipolar Junction Transistors (BJT's), power MOSFETs or IGBTs, are basically DC switches. When they are turned on, they can conduct current in one direction, in compliance with the DC source. A power MOSFET has an intrinsic diode in the reverse direction. Thus it conducts current like a forward biased diode in the reverse direction and does not effectively block voltages in the reverse direction. An IGBT and a BJT have similar limitations, in that they have relatively low breakdown (BD) voltages in the reverse direction from about 12v to about 40v. The limitation of these transistors is explained in U.S. Pat. No. 5,851,857, which is incorporated by reference.
Accordingly, the current use of the transistors in switching power supply circuits is limited to switching DC. Typically, such power conversion systems are DC to DC converters, e.g. choppers or inverters, where the transistors are used in switching operation to convert DC electrical power to AC power, possibly of different frequency than that of the AC line power source.
FIG. 1 illustrates a power supply 100 including a pulse width modulation (PWM) inverter 102. The PWM inverter includes transistors that are used as switches in a PWM method. As explained above, such a transistor operates off a DC power source. This requires the rectification of the AC power source to DC. A rectifier 104 is used for this purpose, i.e., to convert AC to DC and then feed the DC to the PWM inverter. A storage capacitor C1 stores the DC power, i.e., serves as a DC energy reservoir to supply the DC power to the PWM inverter.
FIG. 2 shows a switching power supply including a power factor correction stage 202 and an inverter 204. The PFC stage 202 includes a bridge rectifier 206 that converts the AC line power source to DC power. One purpose of the PFC stage is to improve the poor AC input current waveforms.
The PFC stage also includes a controller 208, a switch 210, and an input filter 212. The switch 210 may comprise a MOSFET or IGBT. The inverter 204 includes a MOSFET switch 222 and a PWM controller 224.
An isolation transformer 226 coupled to the inverter outputs higher frequency electrical power. An output rectifier 228 receives the higher frequency power output by the transformer. An output filter 230 receive the output of the rectifier 228 and outputs DC power.
FIG. 3 illustrates a PFC stage 301 provided in a boost converter 300 (or power supply). The PFC stage includes a transistor T2, an inductor L1, a fast recovery diode D1, and a storage capacitor C2. A controller 302 is coupled to the control terminal of the transistor T2 and is configured to control the output of the PFC stage.
A rectifying stage 304 is provided between an AC input 306 and the PFC stage 301. The PFC stage receives the DC power from the rectifying stage and outputs the DC power to the inverter (not shown) with a boosted voltage. Regular power transistors configured to handle DC power are provided on the side of the inverter. The PFC stage is included in the boost converter in part to resolve the power factor problem associated with the input rectifying stage with its high DC voltage storage capacitor. This extra stage increase the number of components used, adds cost, increases the power dissipation, and reduces the efficiency.
FIG. 4 illustrates motor control circuit 400 with an input rectifying stage 402. The output of the PWM inverter is connected to a corresponding motor. The rectifying stage 402 converts AC from a three-phase AC line to DC, which is then fed to switching circuits 404 including a plurality of transistors.
As described above, the transistors in the conventional power supply circuits are configured to handle DC power not AC power. Accordingly, a rectifying circuit is provided between the transistors and the AC power line. However, it is desirable to operate directly off the AC line if possible. It would be desirable to provide a power supply circuit that is configured to operate directly off the AC power line using one or more power transistors.